1. Field of the Invention
The present invention relates generally to a voltage controlled oscillator (VCO) and semiconductor integrated circuit (IC) device, and more particularly, to a compact voltage controlled oscillator comprising a field-effect transistor (FET) IC.
2. Description of the Related Art
Recent developments in the field of mobile communications devices, in particular cellular telephones, mobile communications equipment and cordless telephones, have resulted in rapid reductions in both the size and price of such equipment. As a result, there is an ever-increasing need to reduce the number of component parts of such devices as well as to reduce the unit costs of such parts. It is for these reasons that the need has arisen to include the voltage controlled oscillator (hereinafter referred to as a VCO) used as a modulation/demodulation circuit in such devices within a complementary metal-oxide semiconductor (CMOS) integrated circuit (IC).
The following can be given as an example of a conventional VCO.
FIG. 1 is a circuit diagram of a conventional VCO 100, in this case a Colpitts type VCO 100 using a bipolar transistor 101. The VCO 100 comprises the bipolar transistor 101 as well as condensers 102, 103, 104, 105, a resistor 108, an inductor 106 and a varicap 107. A collector of the transistor 101 is connected to one terminal of the resistor 108, the other terminal of the resistor 108 being connected to a power source 109.
An emitter of the transistor 101 is connected to one terminal of condenser 102. The other terminal of the condenser 102 is connected to one terminal of condenser 103 and a base of transistor 101. One terminal of condenser 104 is connected to the base of transistor 101, the other terminal of condenser 104 being connected to one terminal of condenser 105 and one terminal of inductor 106. The other terminal of inductor 106 is connected to a ground 110. The other terminal of condenser 105 is connected to a cathode of varicap 107, an anode of varicap 107 being connected to the ground 110. A control voltage 111 is applied at the point of connection between the condenser 105 and the cathode of the varicap 107. By inputting a control voltage 111 at the junction between the condenser 105 and the varicap 107 a voltage applied to the varicap 107 is controlled, the capacitance of the varicap 107 is changed and the oscillation frequency is changed.
FIG. 2 is a diagram for the purpose of describing a conventional CMOS ring-type VCO 200, in which the CMOS inverters are connected in a ring. An NMOS transistor 201 and a PMOS transistor 204 form the first inverter, an NMOS transistor 202 and a PMOS transistor 205 form the second inverter, and an NMOS transistor 203 and a PMOS transistor 206 form the third inverter. An output 211 of the first inverter formed by the NMOS transistor 201 and the PMOS transistor 204 is connected to a gate of NMOS transistor 202, an output 212 of the second inverter formed by the NMOS transistor 202 and the PMOS transistor 205 is connected to a gate of NMOS transistor 202, and an output 210 of the third inverter formed by the NMOS transistor 203 and the PMOS transistor 206 is connected to a gate of NMOS transistor 201, thus connecting in a shape of a ring.
The gates of PMOS transistors 204, 205, 206 are jointly connected and are controlled by a control voltage 209. The current flowing through PMOS transistors 204, 205, 206 is controlled according to the value of the control voltage 209, thus controlling the extent of the delay of each of the inverters connected in the ring and controlling the oscillation frequency.
FIG. 3 is a diagram for the purpose of describing a conventional CMOS inverter VCO, in which the VCO 300 uses CMOS inverters. The VCO 300 comprises an inverter 301, resistor 302, condenser 303, crystal resonator 304, condenser 305, varicap 306 and resistor 307. The resistor 302 and crystal resonator 304 are connected to an input terminal and an output terminal of the inverter 301. The condenser 303 is connected between the output terminal of the inverter 301 and the ground. One terminal of the condenser 305 is connected to the input terminal of the inverter 301 and the other terminal of the condenser 305 is connected to a cathode of the varicap 306. An anode of the varicap 306 is connected to the ground.
One terminal of the resistor 307 is connected to the point of connection between the condenser 305 and the varicap 306, a control voltage 308 being applied to the other terminal of the resistor 307. By controlling the voltage applied to the varicap 306 from the control voltage 308, the capacitance of the varicap 306 is changed and the oscillation frequency of the output signal of the inverter 309 is changed as well.
However, the conventional voltage controlled oscillators described above have the following problem.
The Colpitts type VCO 100 using the bipolar transistor 101 shown in FIG. 1 uses the varicap diode 107. Forming this varicap diode 107 on a semiconductor chip would require a large surface area and would make large-scale integration impractical. Additionally, the varicap diode 107 is difficult to form using the CMOS process widely used for current logic circuits. That is, without using a mixed bipolar/CMOS process the VCO 100 cannot be formed on the same chip as the logic circuit. As a result, in order to form the VCO 100 on the same semiconductor chip as the logic circuit, a mixed bipolar/CMOS process is used. However, such a mixed bipolar/CMOS process complicates the production process and increases the cost of the IC so produced.
Additionally, the CMOS ring-type VCO 200 shown in FIG. 2, in which an odd number of individual CMOS inverters are connected in rings, has the following problem.
The individual NMOS transistors 201, 202, 203 basically operate at saturation, so a rectangular oscillating wave is the form of the oscillator output 210. Since the wave form is a rectangular wave, distortion is high and the carrier-to-noise ratio (C/N) is low.
Moreover, the VCO 300 using the CMOS inverter 301 shown in FIG. 3 has the following problem.
The VCO 300 shown in FIG. 3 uses the crystal resonator 304. As a result, changing the control voltage 308 from a minimum value near ground level to a maximum value near the supply voltage only changes the frequency by about 10 kHz, so the operating range is narrow.